This invention relates to a speed control apparatus of an AC motor and in particular to improvement in characteristic in a higher-speed area than rated speed.
In current control of an AC motor, often vector control is performed wherein the current of the AC motor is disassembled into an excitation component (which will be hereinafter referred to as d axis) and a torque component (which will be hereinafter referred to as q axis), of components on rotating Cartesian two-axis coordinates (which will be hereinafter referred to as dq-axis coordinates) and the components are controlled separately. The case of an induction motor will be discussed below as a related art.
FIG. 16 is a drawing to show the configuration of a speed control apparatus of an induction motor in a related art. In the figure, numeral 31 denotes an induction motor, numeral 32 denotes a PWM inverter for supplying electric power to the induction motor 31 based on voltage command Vu*, Vv*, Vw* described later, numerals 33a, 33b, and 33c denote current detectors for detecting currents iu, iv, and iw of the induction motor 31, and numeral 34 denotes a speed detector for detecting rotation speed xcfx89r of the induction motor 31. Numeral 35 denotes a secondary magnetic flux calculator for calculating magnetic flux xcfx862d based on d-axis current i1d described later, numeral 36 denotes a slip frequency calculator for calculating slip angular frequency xcfx89s based on q-axis current i1q described later and the magnetic flux xcfx862d, numeral 37 denotes a coordinate rotation angular speed calculator for calculating rotation angular speed xcfx89 of dq-axis coordinates based on the slip angular frequency xcfx89s calculated by the slip frequency calculator 36 and the rotation speed xcfx89r of the induction motor 31 detected by the speed detector 34, and numeral 38 denotes an integrator for integrating the rotation angular speed xcfx89 and outputting phase angle xcex8 of dq-axis coordinates. Numeral 39 denotes a three-phase to two-phase coordinate converter for disassembling the currents iu, iv, and iw of the current detectors 33a, 33b, and 33c into the d-axis current i1d and the q-axis current i1q on the dq-axis coordinates based on the phase angle xcex8 of the dq-axis coordinates and outputting the d-axis current i1d and the q-axis current i1q.
Numeral 40 denotes a subtracter for outputting magnetic flux deviation ef between magnetic flux command xcfx862d* and the magnetic flux xcfx862d output by the secondary magnetic flux calculator 35, numeral 41 denotes a magnetic flux controller for controlling proportional integration (which will be hereinafter referred to as PI) so that the magnetic flux deviation ef becomes 0 and outputting d-axis current component iidxe2x80x2, numeral 42 denotes a subtracter for outputting speed deviation ew between speed commands xcfx89r* and the rotation speed xcfx89r of the induction motor 31 output by the speed detector 34, and numeral 43 denotes a speed controller for controlling PI so that the speed deviation ew becomes 0 and outputting q-axis current component i1qxe2x80x2.
Numeral 44 denotes a subtracter for outputting current deviation e1d between d-axis current command i1d* and d-axis current i1d, numeral 45b denotes a d-axis current controller for controlling PI so that the current deviation e1d becomes 0 and outputting d-axis voltage component Vdxe2x80x2, numeral 46 denotes a subtracter for outputting current deviation e1q between q-axis current command i1q* and q-axis current i1q, numeral 47b denotes a q-axis current controller for controlling PI so that the current deviation e1q becomes 0 and outputting q-axis voltage component Vqxe2x80x2, and numeral 48 denotes a two-phase to three-phase coordinate converter for converting d-axis voltage command Vd* and q-axis voltage command Vq* into the voltage commands Vu*, Vv*, and Vw* on the three-phase AC coordinates based on the phase angle xcex8 of the dq-axis coordinates and outputting the voltage commands as voltage commands of the PWM inverter 32.
Numeral 51 denotes a d-axis current limiter for limiting the d-axis current component i1dxe2x80x2 within a predetermined range and outputting the d-axis current command i1d*, and numeral 52 denotes a q-axis current limiter for limiting the q-axis current component i1qxe2x80x2 within a predetermined range and outputting the q-axis current command i1q*. Numeral 53b denotes a d-axis voltage limiter for limiting the d-axis voltage component Vdxe2x80x2 within a predetermined range and outputting the d-axis voltage command Vd*, and numeral 54b denotes a q-axis voltage limiter for limiting the q-axis voltage component Vqxe2x80x2 within a predetermined range and outputting the q-axis voltage command Vq*.
Numeral 55 denotes a magnetic flux command generation section for arbitrarily giving the magnetic flux command xcfx862d* of the induction motor. The speed command xcfx89r* is given arbitrarily from the outside.
FIG. 17 is a drawing to show the configuration of the PI controller of the magnetic flux controller 41, the speed controller 43, the d-axis current controller 45b, the q-axis current controller 47b, etc., in FIG. 16. In FIG. 17, numeral 61 denotes a coefficient unit corresponding to proportional gain KP of the PI controller, numeral 62 denotes a coefficient unit corresponding to integration gain KI of the PI controller, numeral 63b denotes an integrator having a function of stopping calculation, and numeral 64 denotes an adder for adding the proportional component and the integration component.
Letter e denotes deviation input to the PI controller and Uxe2x80x2 denotes control input output from the PI controller. As for the magnetic flux controller 41, e corresponds to the magnetic flux deviation ef between the magnetic flux command xcfx86d2* and the magnetic flux xcfx862d output by the secondary magnetic flux calculator 35, and Uxe2x80x2 corresponds to the d-axis current component i1dxe2x80x2. As for the speed controller 43, a corresponds to the speed deviation ew between the speed command xcfx89r* and the rotation speed xcfx89r of the induction motor 31 output by the speed detector 34, and Uxe2x80x2 corresponds to the q-axis current component i1qxe2x80x2. As for the d-axis current controller 45b, e corresponds to the current deviation eid between the d-axis current command i1d* and the d-axis current i1d, and Uxe2x80x2 corresponds to the d-axis voltage component Vdxe2x80x2. As for the q-axis current controller 47b, e corresponds to the current deviation eiq between the q-axis current command i1q* and the q-axis current i1q, and Uxe2x80x2 corresponds to the q-axis voltage component Vqxe2x80x2.
The basic operation of the vector control in the induction motor will be discussed with FIGS. 16 and 17.
As shown in FIG. 16, the vector control is implemented using a plurality of PI controllers of the magnetic flux controller 41, the speed controller 43, the d-axis current controller 45b, the q-axis current controller 47b, etc., in combination.
The subtracter at the stage preceding each PI controller (subtracter 40, subtracter 42, subtracter 44, subtracter 46) outputs deviation e (ef, ew, eid, eiq) from the command value and actually detected value.
The PI controller is a controller for setting the deviation output from the subtracter at the predetermined stage to 0 (matching the command value and actually detected value with each other). Each PI controller inputs the deviation e output from the subtracter at the preceding stage and outputs such control input Uxe2x80x2 (i1dxe2x80x2, i1qxe2x80x2, Vdxe2x80x2, Vqxe2x80x2) setting the deviation e to 0 based on the following expression (1):
Uxe2x80x2=(KP+(KI/s))3xc2x7e xe2x80x83xe2x80x83(1) 
The block diagram of expression (1) is shown in FIG. 17, wherein KP denotes the proportional gain of the PI controller and KI denotes the integration gain of the PI controller. The PI controller used in FIG. 16 (magnetic flux controller 41, speed controller 43, d-axis current controller 45b, q-axis current controller 47b) is the PI controller shown in FIG. 17, but the PI controllers differ in values of KP and KI.
In the magnetic flux controller 41 or the speed controller 43, the d-axis current component i1dxe2x80x2 or the q-axis current component i1qxe2x80x2 corresponds to the control input Uxe2x80x2, but cannot be set to a value equal to or greater than maximum output current value imax allowed by the PWM inverter 32. Then, the d-axis current limiter 51, the q-axis current limiter 52 limits so that the control input Uxe2x80x2 output from the magnetic flux controller 41, the speed controller 43 (d-axis current component i1dxe2x80x2, q-axis current component, i1qxe2x80x2) does not exceed the maximum output current value imax allowed by the PWM inverter 32.
In the d-axis current controller 45b or the q-axis current controller 47b, the d-axis voltage component Vdxe2x80x2 or the q-axis voltage component Vqxe2x80x2 corresponds to the control input Uxe2x80x2, but cannot be set to a value equal to or greater than bus voltage VDC of the PWM inverter 32. Thus, the d-axis voltage limiter 53b, the q-axis voltage limiter 54b limits so that the control input Uxe2x80x2 output from the d-axis current controller 45b or the q-axis current controller 47b (d-axis voltage component Vdxe2x80x2 or q-axis voltage component Vqxe2x80x2) does not exceed the bus voltage VDC of the PWM inverter 32.
However, the limit values of the d-axis current limiter 51, the q-axis current limiter 52, the d-axis voltage limiter 53b, and the q-axis voltage limiter 54b need not necessarily be the same.
As described above, in the speed control apparatus of the induction motor in the related art, the limiters 51, 52, 53b, and 54b are provided for outputs of the PI controllers 41, 43, 45b, and 47b and if the control input Uxe2x80x2 is limited by the limiter 51, 52, 53b, 54b, input deviation e does not become 0 for ever and if the deviation e continues to be accumulated in the integrator 63b in the PI controller, a phenomenon called control input saturation arises, causing a vibratory output response called overshoot or hunting; this is a problem.
Thus, if the control input Uxe2x80x2 exceeds the limit value of the limiter 51, 52, 53b, 54b, empirically the integration operation of the integrator 63b in the PI controller is stopped, thereby avoiding continuing to accumulator the deviation e for eliminating control input saturation, thereby obtaining a stable response.
FIG. 18 is a graph plotting the d-axis voltage component Vdxe2x80x2 and the q-axis voltage component Vqxe2x80x2 based on expressions for finding terminal-to-terminal voltage in a stationary state in the induction motor described later: In the figure, (a), (c), and (e) indicate the d-axis voltage component Vdxe2x80x2 and (b), (d), and (f) indicate the q-axis voltage component Vqxe2x80x2.
FIG. 19 is a graph to show the limit values of the q-axis current limiter relative to the rotation speed xcfx89r.
FIG. 20 is a graph to show the maximum allowable values of the magnetic flux command xcfx862d* that can be arbitrarily output from the magnetic flux command generation section relative to the rotation speed xcfx89r.
FIG. 18 corresponds to FIGS. 19 and 20. If the limit value is changed to (a), (c), and (e) in FIG. 19, the graph of FIG. 18 becomes as (a), (c), and (e). If the maximum allowable value is changed to (b), (d), and (f) in FIG. 20, the graph of FIG. 18 becomes as (b), (d), and (f).
To operate the induction motor at rated speed or more, the d-axis voltage component Vdxe2x80x2 and the q-axis voltage component Vqxe2x80x2 output from the d-axis current controller 45b and the q-axis current controller 47b continue to exceed the limit values of the d-axis voltage limiter 53b and the q-axis voltage limiter 54b stationarily. The above-described method of stopping the integration operation if the control input exceeds the limit value is means for temporarily avoiding the uncontrollable state of control input saturation and is effective for transient control input saturation, but cannot be used if control input saturation continues to occur stationarily as whether the induction motor is operated at the rated speed or more.
A method in related art for eliminating control input saturation of the voltage components Vdxe2x80x2 and Vqxe2x80x2 occurring stationarily at the rate speed or more will be discussed with FIGS. 18 to 20. Such control input saturation of Vdxe2x80x2 and Vqxe2x80x2 in high-speed area is particularly called voltage saturation.
As for the induction motor, the d-axis voltage component Vdxe2x80x2 and the q-axis voltage component Vqxe2x80x2 in a stationary state are given according to the following expressions (2) and (3):
Vdxe2x80x2=R1xc2x7i1dxe2x88x92L1xc2x7"sgr"xc2x7xcfx89xc2x7i1q xe2x80x83xe2x80x83(2) 
Vqxe2x80x2=R1xc2x7i1q+(L1/M)xc2x7xcfx89xc2x7xcfx862d xe2x80x83xe2x80x83(3) 
where R1 denotes primary resistance of the induction motor 31, L1 denotes primary side self inductance, M denotes mutual inductance, and "sgr" denotes a leakage coefficient.
To operate the induction motor at the rated speed or more, the second term component in expression (2), (3) becomes very larger than the first term component and thus expression (2) and (3) can be approximated by the following expressions (4) and (5):
Vdxe2x80x2=xe2x88x92L1xc2x7"sgr"xc2x7xcfx89xc2x7i1q xe2x80x83xe2x80x83(4) 
Vqxe2x80x2=(L1/M)xc2x7xcfx89xc2x7xcfx862d xe2x80x83xe2x80x83(5) 
The q-axis current limiter 52 is a fixed limiter and the q-axis current limiter value is indicated by FIG. 19 (a). Here, assuming that the q-axis current i1q flows as much as the limit value, Vdxe2x80x2 becomes the graph of FIG. 18 (a) according to expression (4). The maximum allowable value of xcfx862d* that can be arbitrarily output from the magnetic flux command generation section 55 is indicated by FIG. 20 (b). Here, assuming that the magnetic flux xcfx862d takes the same value as the maximum allowable value, Vqxe2x80x2 becomes the graph of FIG. 18 (b) according to expression (5).
From FIGS. 18 (a) and (b), it is seen that to operate the induction motor at rotation speed xcfx89base or more, the voltage component Vqxe2x80x2 becomes saturated exceeding the output limit value of the PWM inverter 32 xc2x1Vmax and that to operate the induction motor at rotation speed xcfx89base2 or more, both the voltage components Vdxe2x80x2 and Vqxe2x80x2 become saturated exceeding the output limit value of the PWM inverter 32 xc2x1Vmax.
Since voltage saturation occurs stationarily in such an area at the rated speed or more, the maximum allowable value of xcfx862d* of the magnetic flux command generation section 55 and the limit value of the q-axis current limiter 52 are changed in response to the speed. For example, if a variable limiter is adopted for changing the limit value of the q-axis current limiter in a manner inversely proportional to the speed from the rotation speed xcfx89base2 at which saturation of the d-axis component occurs as indicated by FIG. 19 (c), even if the q-axis current i1q flows as much as the limit value, Vdxe2x80x2 becomes the graph of FIG. 18 (c) according to expression (4). If the maximum allowable value of xcfx862d* that can be arbitrarily output from the magnetic flux command generation section 55 is limited by a function inversely proportional to the speed from the rotation speed xcfx89base at which voltage saturation of the q-axis component occurs as indicated by FIG. 20 (d), even if the magnetic flux xcfx862d takes the same value as the maximum allowable value, Vqxe2x80x2 becomes the graph of FIG. 18 (d) according to expression (5).
As described above, the limit value of the q-axis current limiter and the maximum allowable value of xcfx862d* are changed in response to the speed, whereby the d-axis voltage component Vdxe2x80x2 and the q-axis voltage component Vqxe2x80x2 are prevented from exceeding the output limit value of the PWM inverter 32 xc2x1Vmax even in an area at the rated speed or more, and occurrence of voltage saturation can be suppressed, so that a stable response can be provided.
However, if the induction motor is actually turned, the voltage component Vdxe2x80x2, Vqxe2x80x2 may become larger than FIG. 18 (c), (d) because of fluctuation of the magnitude of load or bus voltage, and voltage saturation occurs, resulting in an unstable response.
Then, the limit value of the q-axis current limiter and the maximum allowable value of xcfx86d2* are set further lower as in FIG. 19 (e) and FIG. 20 (f) and the voltage component Vdxe2x80x2, Vqxe2x80x2 can be provided with a margin relative to the output limit value of the RWM inverter 32 xc2x1Vmax as in FIGS. 18 (e), (f) for making voltage saturation hard to occur.
In this case, however, it is made impossible to make full use of the capabilities of the PWM inverter and lowering of output torque or the like is incurred; this is a problem.
To make voltage saturation hard to occur without lowering the output torque, a method of feeding back a magnetic flux command or a current command for correction if voltage saturation occurs is proposed. In this method, when voltage saturation occurs, the saturation amount is detected, an optimum correction amount to eliminate the voltage saturation is formed from the saturation amount, and each command is corrected. Such feedback control is performed, whereby occurrence of voltage saturation can be suppressed and stability of control can be improved independently of the conditions of the load and the bus voltage, and it is also made possible to make full use of the capabilities of the PWM inverter.
For example, the Unexamined Japanese Patent Application No. 2000-92899 discloses a control apparatus of an induction motor, comprising a voltage saturation compensation circuit for making a comparison between a voltage command value from a current control system and a bus voltage value of a PWM inverter and integrating and if the above-mentioned bus voltage value is greater than the above-mentioned voltage command value, the voltage saturation compensation circuit for subtracting the above-mentioned integrated output from a magnetic flux command and if the above-mentioned bus voltage value is lower than the above-mentioned voltage command value, for subtracting 0 from the magnetic flux command.
In this method, the correction amount is derived in response to the voltage saturation amount and each command is corrected, so that voltage saturation can be eliminated; however, since the speed of the motor is not considered when the correction amount is determined, to cope with rapid speed change, etc., calculation of the correction amount, etc., must be thought out in such a manner that the correction amount is increased to make a prompt correction at the acceleration time and that the correction amount is suppressed to raise stability at the deceleration time, for example; the method involves such a problem.
The invention is intended for solving the problems as described above and it is an object of the invention to provide a speed control apparatus of an AC motor for making it possible to suppress occurrence of voltage saturation without performing special operation even if rapid speed change, etc., occurs.
According to the invention, there is provided a speed control apparatus of an AC motor having current controllers for performing proportional integration control of an excitation component current and a torque component current of two components on rotating Cartesian two-axis coordinates into which a current of the AC motor is separated, the speed control apparatus comprising:
a torque component voltage limiter for limiting a torque component voltage component output from torque component current controller for performing proportional integration control of the torque component current so that the torque component voltage component becomes equal to or less than a predetermined value; a first subtracter for finding a torque component voltage saturation amount from the torque component voltage component output from the above-mentioned torque component current controller and a torque component voltage command output from the above-mentioned torque component voltage limiter; a first integrator for holding the torque component voltage saturation amount; a magnetic flux command corrector for outputting a magnetic flux command correction amount from the held torque component voltage saturation amount and rotation angular speed of Cartesian two-axis coordinates; and a second subtracter for subtracting the magnetic flux command correction amount from a magnetic flux command and outputting a magnetic flux correction command, so that if the speed rapidly changes, etc., the optimum correction amount can always be obtained and occurrence of voltage saturation can be suppressed.
Also, there is provided a speed control apparatus of an AC motor having current controllers for performing proportional integration control of an excitation component current and a torque component current of two components on rotating Cartesian two-axis coordinates into which a current of the AC motor is separated, the speed control apparatus comprising:
a torque component voltage limiter for limiting a torque component voltage component output from torque component current controller for performing proportional integration control of the torque component current so that the torque component voltage component becomes equal to or less than a predetermined value; a first subtracter for finding a torque component voltage saturation amount from the torque component voltage component output from the above-mentioned torque component current controller and a torque component voltage command output from the above-mentioned torque component voltage limiter; a first integrator for holding the torque component voltage saturation amount; an excitation component current command corrector for outputting an excitation component current command correction amount from the held torque component voltage saturation amount and rotation angular speed of Cartesian two-axis coordinates; and a third subtracter for subtracting the excitation component current command correction amount from an excitation component current command and outputting an excitation component current command correction command, so that
if the speed rapidly changes, etc., the optimum correction amount can always be obtained and it is possible to suppress occurrence of voltage saturation.
Rotation speed of the above-mentioned AC motor is input to a magnetic flux command generation section for generating a magnetic flux command and a magnetic flux command is generated in response to the rotation speed of the above-mentioned AC motor, so that the magnitude of the torque component voltage saturation amount can be lessened to some extent and it is made possible to improve the stability of control of the AC motor.
Rotation speed of the above-mentioned AC motor is input to an excitation component current command generation section for generating an excitation component current command and an excitation component current command is generated in response to the rotation speed of the above-mentioned AC motor, so that the magnitude of the torque component voltage saturation amount can be lessened to some extent and it is made possible to improve the stability of control of the AC motor.
The speed control apparatus comprises an excitation component voltage limiter for limiting an excitation component voltage component output from excitation component current controller for performing proportional integration control of the excitation component current so that the excitation component voltage component becomes equal to or less than a predetermined value; a fourth subtracter for finding the excitation component voltage component output from the above-mentioned excitation component current controller and an excitation component voltage saturation amount output from the above-mentioned excitation component voltage limiter; a second integrator for holding the excitation component voltage saturation amount; an excitation component current command corrector for outputting a torque component current command correction amount from the held excitation component voltage saturation amount and rotation angular speed of Cartesian two-axis coordinates; and a fifth subtracter for subtracting the torque component current command correction amount from a torque component current command and outputting a torque component current correction command, so that to operate the AC motor in an area wherein the speed largely exceeds the rated speed, occurrence of voltage saturation can also be suppressed and it is possible to perform stable control.
In a torque component current limiter for limiting a torque component current command output from a speed controller for performing proportional integration control of speed deviation between a speed command and the rotation speed of the AC motor so that the torque component current command becomes equal to or less than a predetermined value, the limit value for limiting the torque component current command is varied in response to the rotation speed of the above-mentioned AC motor, so that the magnitude of the extinction component voltage saturation amount can be lessened to some extent and it is made possible to improve the stability of control of the AC motor.
According to the invention, there is provided a speed control apparatus of an AC motor having current controllers for performing proportional integration control of an excitation component current and a torque component current of two components on rotating Cartesian two-axis coordinates into which a current of the AC motor is separated, wherein torque component current controller for performing proportional integration control of the torque component current is configured so as to continue calculation of an internal integrator even if torque component voltage component becomes saturated, the speed control apparatus comprising a torque component voltage limiter for limiting the torque component voltage component output from the torque component current controller for performing proportional integration control of the torque component current so that the torque component voltage component becomes equal to or less than a predetermined value; a first subtracter for finding a torque component voltage saturation amount from the torque component voltage component output from the above-mentioned torque component current controller and a torque component voltage command output from the above-mentioned torque component voltage limiter; a magnetic flux command corrector for outputting a magnetic flux command correction amount from the torque component voltage saturation amount and rotation angular speed of Cartesian two-axis coordinates; and a second subtracter for subtracting the magnetic flux command correction amount from a magnetic flux command and outputting a magnetic flux correction command, so that
occurrence of voltage saturation can be suppressed according to the sample configuration.
Also, there is provided a speed control apparatus of an AC motor having current controllers for performing proportional integration control of an excitation component current and a torque component current of two components on rotating Cartesian two-axis coordinates into which a current of the AC motor is separated, wherein
torque component current controller for performing proportional integration control of the torque component current is configured so as to continue calculation of an internal integrator even if torque component voltage component becomes saturated, the speed control apparatus comprising a torque component voltage limiter for limiting a torque component voltage component output from torque component current controller for performing proportional integration control of the torque component current so that the torque component voltage component becomes equal to or less than a predetermined value; a first subtracter for finding a torque component voltage saturation amount from the torque component voltage component output from the above-mentioned torque component current controller and a torque component voltage command output from the above-mentioned torque component voltage limiter; an excitation component current command corrector for outputting an excitation component current command correction amount from the torque component voltage saturation amount and rotation angular speed of Cartesian two-axis coordinates; and a third subtracter for subtracting the excitation component current command correction amount from an excitation component current command and outputting an excitation component current command correction command, so that
occurrence of voltage saturation can be suppressed according to the simple configuration.
Rotation speed of the above-mentioned AC motor is input to a magnetic flux command generation section for generating a magnetic flux command and a magnetic flux command is generated in response to the rotation speed of the above-mentioned AC motor, so that the magnitude of the torque component voltage saturation amount can be lessened to some extent and it is made possible to improve the stability of control of the AC motor according to the simple configuration.
Rotation speed of the above-mentioned AC motor is input to an excitation component current command generation section for generating an excitation component current command and an excitation component current command is generated in response to the rotation speed of the above-mentioned AC motor, so that the magnitude of the torque component voltage saturation amount can be lessened to some extent and it is made possible to improve the stability of control of the AC motor according to the simple configuration.
Excitation component current controller for performing proportional integration control of the excitation component current is configured so as to continue calculation of an internal integrator even if excitation component voltage component becomes saturated, and the speed control apparatus comprises an excitation component voltage limiter for limiting the excitation component voltage component output from the excitation component current controller for performing proportional integration control of the excitation component current so that the excitation component voltage component becomes equal to or less than a predetermined value; a fourth subtracter for finding the excitation component voltage component output from the above-mentioned excitation component current controller and an excitation component voltage saturation amount output from the above-mentioned excitation component voltage limiter; an excitation component current command corrector for outputting a torque component current command correction amount from the excitation component voltage saturation amount and rotation angular speed of Cartesian two-axis coordinates; and a fifth subtracter for subtracting the torque component current command correction amount from a torque component current command and outputting a torque component current correction command, so that
to operate the AC motor in an area wherein the speed largely exceeds the rated speed, occurrence of voltage saturation can also be suppressed and it is possible to perform stable control according to the simple configuration.
In a torque component current limiter for limiting a torque component current command output from a speed controller for performing proportional integration control of speed deviation between a speed command and the rotation speed of the AC motor so that the torque component current command becomes equal to or less than a predetermined value, the limit value for limiting the torque component current command is varied in response to the rotation speed of the above-mentioned AC motor, so that the magnitude of the excitation component voltage saturation amount can be lessened to some extent and it is made possible to improve the stability of control of the AC motor according to the simple configuration.